This study is evaluating the pharmacokinetic interaction between clarithromycin, rifabutin, and ethambutol in HIV-infected patients with Mycobacterium avium infection being treated with combination chemotherapy as a part of an NIH-sponsored clinical trial (ACTG Protocol 823). Dr. Flexner is the national Protocol Chair for this study. To date, 11 patients have completed this protocol nationally, 8 of them at Johns Hopkins. This study is ongoing, and is being modified to incorporate the HIV protease inhibitors ritonavir and indinavir, which are expected to undergo significant pharmacokinetic interaction with the agents already being studied in this protocol. All pharmacokinetic drug level analysis is being performed in the Drug Assay Laboratory of our Division of Clinical Pharmacology in Osler 500.